Superalloys are widely used as castings in the gas turbine engine industry for critical components, such as turbine airfoils including blades and vanes, subjected to high temperatures and stress levels. Such critical components oftentimes are cast using well known directional solidification (DS) techniques that provide a single crystal microstructure or columnar grain microstructure to optimize properties in one or more directions.
Directional solidification casting techniques are well known wherein a nickel base superalloy remelt ingot is vacuum induction remelted in a crucible in a casting furnace and poured into a ceramic investment cluster mold disposed in the furnace having a plurality of mold cavities. During directional solidification, the superalloy melt is subjected to unidirectional heat removal in the mold cavities to produce a columnar grain structure or single crystal in the event a crystal selector or seed crystal is incorporated in the mold cavities. Unidirectional heat removal can be effected by the well known mold withdrawal technique wherein the melt-filled cluster mold on a chill plate is withdrawn from the casting furnace at a controlled rate. Alternately, a power down technique can be employed wherein induction coils disposed about the melt-filled cluster mold on the chill plate are de-energized in controlled sequence. Regardless of the DS casting technique employed, generally unidirectional heat removal is established in the melt in the mold cavities.
Since single crystal castings do not include grain boundaries, prior art workers believed that elements, such as carbon and boron, that from grain boundary strengthening precipitates in the microstructure would not be necessary in single crystal superalloy compositions.
However, U.S. Pat. No. 5,549,765 describes a nickel base superalloy having increased carbon concentration to produce a cleaner casting. Although the nickel base superalloy of the '765 patent improves alloy cleanliness and castability, a reduction in mechanical properties, such as stress rupture life, at elevated temperatures, such as at and above 1400° F., has been observed in laboratory testing.